Such contactlessly working measuring devices are applied in a large number of branches of industry, e.g. in the processing industry, in the chemicals industry or in the foods industry.
Typically, the fill level measuring device is mounted above the fill substance and the antenna of the device is directed toward the fill substance.
For determining travel time, all known methods can be applied, which enable relatively short distances to be measured by means of reflected microwaves. Best known examples are pulse radar and frequency modulation continuous wave radar (FMCW radar).
In the case of pulse radar, short microwave transmission pulses, referred to in the following as wave packets, are transmitted periodically, reflected by the surface of the fill substance and received back after a distance dependent travel time. On the basis of the received signal, an echo function is derived, which shows the received signal amplitude as a function of time. Each value of this echo function corresponds to the amplitude of an echo reflected at a certain distance from the antenna.
In the FMCW method a microwave signal, which is periodically linearly frequency modulated, for example, according to a saw tooth function, is continuously transmitted. Consequently, the frequency of the received echo signal has, compared to the instantaneous frequency of the transmission signal at the point in time of receipt, a frequency difference, which depends on the travel time of the microwave signal and its echo signal. The frequency difference between the transmission signal and the received signal can be gained by mixing both signals and evaluating the Fourier spectrum of the mixed signal. Then, from the frequency difference, the distance of the reflecting surface from the antenna can be calculated. Additionally, the amplitudes of the spectral lines of the frequency spectrum gained by Fourier transformation correspond to the echo amplitudes. This Fourier spectrum consequently represents, in this case, the echo function.
From the echo function, at least one wanted echo is determined, which corresponds to the reflection of the transmission signal off the surface of the fill substance. From travel time of the wanted echo, there directly results, in the case of a known propagation velocity of the microwaves, the distance which the microwaves travel on their way from the measuring device to the surface of the fill substance and back. On the basis of the installed height of the fill level measuring device over the container, fill level can then be directly calculated.
Today's fill level measuring devices have, as a rule, an antenna designed for a predetermined frequency or a predetermined, narrow, frequency band.
There are, however, a large number of applications, in the case of which it is advantageous to perform fill level measurements with markedly different frequencies. In order to enable this, as a rule, two separate fill level measuring devices in each case designed for a frequency or a predetermined frequency band, or at least two separate antennas in each case designed for a frequency or a predetermined frequency band, are applied. In modern industrial plants, a large number of different measurements are, as a rule, performed in the individual containers. For this, a large number of different measuring devices are applied at, on or in the respective container. For this purpose, however, only a limited number of appropriate container openings and/or mounting apparatuses are present. Accordingly, it is frequently not possible or not desirable to provide for fill level measurements two or more fill level measuring devices or two or more antennas on a single container.